The invention relates to an amplification circuit and more particularly to an amplification circuit which includes a device for compensating for its input current.
The invention will be more particularly described within the framework of the restoration of the DC component of a video signal.
However, as will emerge later, the invention relates to other applications such as, for example the stabilization of a voltage.
As is known to those skilled in the art, the DC component of the video signal delivered by an image sensor is not transmitted by the processing circuits located on the output side of the image sensor. A circuit for restoring the DC component of a video signal is therefore necessary.
The circuit in FIG. 1 represents a diagram showing the principle of a circuit for restoring the DC component of a video signal according to the prior art.
The circuit in FIG. 1 includes a capacitor C, commonly called a clamping capacitor, a switch K and an amplifier A1.
The switch K has an on-state resistance R.sub.ON, the value of which is typically less than or equal to 100 and an off-state resistance of virtually infinite value.
The video signal VE is applied to a first plate of the capacitor C, the second plate of which is connected to the input of the amplifier A1. A first terminal of the switch K is connected to the common point connecting the second plate of the capacitor C and the input of the amplifier A1, and a second terminal of the switch K is connected to the earth of the circuit. As is known to those skilled in the art, the output voltage VS1 of the amplifier A1 is preferably equal to zero volts.
When the switch K is closed, the second plate of the capacitor is earthed, to the earth of the circuit, with the time constant .tau.=C.times.R.sub.ON As is known to those skilled in the art, the switch K is closed by the action of a control pulse generated from a sync signal.
The switch is closed for a time T lying within the line blanking interval, after the trailing edge of the sync signal. The time .tau. is adjusted so as to be very much shorter than the time T, which is about 3 to 4 .mu.s. It follows that the input voltage of the amplifier A1 is clamped to earth at each line, whatever the value of the potential present on the input of the amplifier A1 at the moment when the switch K closes.
Between two line sync pulses, the capacitor C behaves as a battery and delivers its input current Ia to the amplifier A1. The variation in voltage .DELTA.V.sub.c which then appears across the terminals of the capacitor C is such that:
.DELTA.V.sub.c =.DELTA.Q/C, with .DELTA.Q=Ia.times.T.sub.L where T.sub.L represents the time separating two line sync pulses.
It follows therefore that: EQU .DELTA.V.sub.c =T.sub.L .times.Ia/C.
In order to minimize the value of .DELTA.V.sub.c, the amplifier A1 is chosen so as to have a high input impedance. It follows that the current Ia is small and that the voltage variation .DELTA.V.sub.c can then be neglected.
Thus, according to the known prior art, the input stage of the amplifier A1 is produced either using one or two junction field-effect transistors (JFET) or using one or two field-effect transistors of the MOSFET type, the JFET and MOSFET transistors having virtually infinite input impedances.
The use of JFET or MOSFET transistors has, however, many drawbacks.
This is because JFET transistors, apart from the fact that they are expensive, have a high consumption, typically about 30 mA, and, for their supply, they require a format of voltages of relatively high value (typically between +12 volts and -12 volts).
With regard to MOSFET transistors, the noise voltage density which they generate is very high, about 20 to 25 nV per Hz, thereby preventing any use of these transistors for professional applications.
The invention does not have such drawbacks.